ignore .xwm generated by xwatermark package in TeX (#3412)

TeX.gitignore

@@ -275,6 +275,9 @@ TSWLatexianTemp*
 # Makeindex log files
 *.lpz
 
+# xwatermark package
+*.xwm
+
 # REVTeX puts footnotes in the bibliography by default, unless the nofootinbib
 # option is specified. Footnotes are the stored in a file with suffix Notes.bib.
 # Uncomment the next line to have this generated file ignored.